When Precursor Golem dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Precursor Golem from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Precursor Golem enters the battlefield, its first ability triggers, creating two 3/3 Golem creature tokens.
Activate Ashnod's Altar by sacrificing a Golem, adding ({C} magic symbol) ({C} magic symbol) .